Transmitters form one half of most communication circuits. As such, they assume a position of prominence in design concerns. With the proliferation of mobile terminals, transmitter design has progressed in leaps and bounds as designers try to minimize components and reduce size, battery consumption, and the like. Likewise, modulation schemes are continuously updated to reflect new approaches to maximize information transfers in limited bandwidths. Changes in standards or standards based on newly available spectra may also cause designers to approach modulating transmitters with different techniques.
Many different standards and modulation schemes exist, but one of the most prevalently used in the world of mobile terminals is the Global System for Mobile Communications (GSM). One of the modulation schemes of the GSM standard is the Enhanced Data Rates for GSM Evolution (EDGE) modulation scheme. The EDGE modulation scheme contains an amplitude modulation component and a phase modulation component. Since there is an amplitude modulation component, the power amplifier of a transmitter operating according to the EDGE modulation scheme must be linear or driven according to a polar modulation scheme.
If a polar modulation scheme is used, a phase modulated signal at the desired radio frequency is provided to the input of the power amplifier and an amplitude modulation component is used to vary the supply voltage provided to the power amplifier. As a result, the power amplifier may operate in saturation and efficiency is greatly improved. Unfortunately, the amplitude modulation component that controls the supply voltage provided to the power amplifier causes unwanted phase components to be created in the output of the power amplifier due to the non-linearities of the power amplifier. This is sometimes called Amplitude Modulation to Phase Modulation (AM/PM) distortion, and it degrades the spectral purity of the system and the Error Vector Magnitude.
In order to compensate for the AM/PM distortion, polar modulation systems may pre-distort the phase modulation component provided to the power amplifier such that the AM/PM distortion at the output of the power amplifier is substantially reduced. As an example, see commonly owned and assigned U.S. Patent Application Publication No. 2003/0215025, entitled AM TO PM CORRECTION SYSTEM FOR POLAR MODULATOR, filed May 16, 2002, which is hereby incorporated by reference in its entirety.
One issue with the AM/PM pre-distortion, or compensation, is that the AM/PM distortion is frequency dependent. As such, circuitry providing the AM/PM pre-distortion must be calibrated for each desired frequency band and preferably each of a number of sub-bands within each of the desired frequency bands in order to effectively reduce the AM/PM distortion. Further, due to variations in the manufacturing process causing variations in the AM/PM distortion between like transmitters, it is desirable to calibrate the AM/PM pre-distortion for each transmitter. Due to the desire to calibrate each transmitter and the number of calibrations desired for each transmitter, there remains a need for accurately and quickly calibrating AM/PM pre-distortion in a transmitter operating according to a polar modulation scheme.